1. Field of the Invention
The present invention relates to an apparatus and method for detecting an interference wave in a desired receiving band in a receiver or the like, which demodulates a received signal using, for example, a fast Fourier transform (FFT) scheme such as Orthogonal Frequency Division Multiplexing (OFDM), and also relates to an equalizer incorporating such apparatus.
2. Description of the Related Art
Conventionally, in the Integrated Services Digital Broadcasting-Terrestrial (ISDB-T) or the like, the transmission path estimation of an OFDM modulated signal (hereinafter referred to as an “OFDM signal”) in a receiver is performed as described in, for example, patent document 1 (Japanese Patent Kokai Publication No. 2000-115087). The apparatus in patent document 1 frequency-converts an OFDM digital baseband signal using FFT; extracts a desired signal such as a scattered pilot (SP) signal in ISDB-T; obtains the amplitude-frequency characteristic and phase-frequency characteristic on the basis of the extracted SP signal; generates a delay profile (electric power relative to a delay time) by inverse-fast-Fourier-transforming (IFFT) the amplitude-frequency characteristic and phase-frequency characteristic; and generates a transmission path estimation signal using the delay profile. Furthermore, the same holds true for an apparatus using a known information signal (pilot signal) other than the SP signal.
In the conventional receiver, when a unique interference wave is mixed into a desired signal wave, an OFDM signal is deformed and therefore the receiving characteristic is degraded. In order to suppress such degradation, the receiver needs to have a means that can detect interference waves.
However, in the conventional method of the transmission path estimation, the presence or absence of the interference wave cannot be detected. Therefore, in the conventional method of the transmission path estimation, when the interference wave power is large, the interference wave power is superimposed on not only subcarriers near the interference wave but also the other subcarriers apart from the interference wave, and therefore the interference wave power exerts an influence upon subcarrier information of the subcarriers around a subcarrier where the interference wave power exits. As a result, the demodulated signal generated by equalizing processing using the transmission path estimation signal as it is, includes an error of the transmission path estimation information itself, and therefore the occurrence frequency of the demodulation errors increases undesirably.
Furthermore, if there is an error correcting encoder such as a viterbi encoder at the later stage, which uses reliability information (which indicates a degree of reliability of a signal obtained from electric power or amplitude of each subcarrier or the like on the basis of the transmission path estimation result generated from a known signal.) as a soft decision result, the subcarrier information of the subcarriers which have been subjected to larger influence of the interference wave, is determined to have higher reliability information undesirably. Accordingly, there is a problem that even if the subcarrier information has high probability of error, it is regarded as reliable information and is input to an error correction encoder, and therefore the error correction performance decreases.